Voltage Drop Calculator Use
This voltage drop calculator helps electricians, engineers, and DIY enthusiasts determine the amount of voltage lost in an electrical circuit. By inputting the system voltage, expected current, wire gauge, and distance, you can ensure your electrical installation meets safety standards and operates efficiently.
Maintaining proper voltage is critical because excessive drop can lead to motor failure, overheating of conductors, and inefficient operation of electronic devices. The National Electrical Code (NEC) recommends a maximum voltage drop of 3% for branch circuits and 5% for the entire system from the service entrance to the farthest outlet.
- Electrical Phase
- Choose between Single Phase (residential standard 120/240V) or Three Phase (commercial/industrial standard 208/480V).
- Conductor Material
- Copper is the standard for most interior wiring, while Aluminum is often used for service entrance cables and large feeders due to lower cost and weight.
- Wire Size (AWG)
- American Wire Gauge (AWG) measures the diameter of the wire. Smaller numbers represent thicker wires with lower resistance.
How the Voltage Drop is Calculated
When electricity flows through a conductor, it encounters resistance. This resistance converts some electrical energy into heat, resulting in a lower voltage at the load compared to the source. The voltage drop calculator uses the standard Ohm's Law derived formulas:
Single Phase: VD = (2 × L × R × I) / 1000
Three Phase: VD = (1.732 × L × R × I) / 1000
- VD: Voltage Drop in Volts.
- L: One-way length of the conductor in feet.
- R: Resistance of the conductor in Ohms per 1,000 feet.
- I: Load current in Amperes (Amps).
- 2: Multiplier for single phase (current travels out and back).
- 1.732: Square root of 3, used for three-phase balanced loads.
Voltage Drop Calculation Example
Scenario: You are running a 120V circuit to a detached garage 150 feet away using 12 AWG Copper wire to power a tool drawing 15 Amps.
Step-by-step solution:
- Determine Resistance: 12 AWG Copper has a resistance of approx 1.93 Ω per 1,000ft.
- Identify Distance: L = 150 feet.
- Identify Current: I = 15 Amps.
- Apply Single Phase Formula: VD = (2 × 150 × 1.93 × 15) / 1000
- Calculate: VD = 8685 / 1000 = 8.685 Volts.
- Determine Percentage: (8.685 / 120) × 100 = 7.24%.
- Result: Since 7.24% exceeds the recommended 3%, you should consider upgrading to 10 AWG or 8 AWG wire.
Common Questions
Why is voltage drop a problem?
Voltage drop causes appliances to run inefficiently. Lights may flicker or appear dim, and electric motors may struggle to start or run hotter than designed, significantly shortening their lifespan. It is also a waste of energy that you pay for but never get to use at the equipment.
Does wire temperature affect voltage drop?
Yes. As a wire gets hotter, its resistance increases. This calculator uses standard resistance values typical for 75°C (167°F) conductor temperatures, which is the standard for most electrical engineering calculations and code requirements.
How do I reduce voltage drop?
The most effective way to reduce voltage drop is to increase the size (diameter) of the wire. Alternatively, you can reduce the load current or shorten the distance of the run, though these are often not possible in practical installations. Switching from aluminum to copper also reduces drop due to copper's lower resistivity.